The present invention pertains to burner components, such as the burner components of an engine or other power source, and more particularly to a fuel injector for a liquid fuel burner.
Burners, such as liquid fuel burners, may be used in a wide range of applications including power sources, heat sources, heating appliances and light sources. Typically, it is desirable to have a burner with properties such as high thermal efficiency and low emissions. One method to achieve low emissions is to mix a fuel with air before burning the fuel in the burner. Liquid hydrocarbons, such as kerosene and heating oil, need to first be evaporated before being mixed with air for burning. The evaporation of fuel in high power burners is traditionally achieved by atomizing the fuel into a fog of droplets that readily evaporate and mix with the combustion air. Liquid fuels are typically atomized by forcing the liquid fuel through a small hole with significant pressure. However, such an approach is typically limited to burner powers above 12 kW. Below this flow rate, good atomization requires impracticably small holes. Small oil heaters typically use wicks to evaporate the fuel and mix it with air. However, it is difficult to turn down a wick burner and it is therefore not a good choice for a burner for a power source such as an engine.
In addition to the limited low flow capabilities, another problem with typical fuel injectors is the coking of the fuel in the injector. Coking is the de-hydrogenation of the liquid hydrocarbon fuel that produces a tar that clogs the fuel injector ports. This is particularly a problem during shut down of a burner when the fuel flow is stopped while the burner is hot. The fuel left in the hot injector bakes and forms tar deposits.
As mentioned, a burner may be used in a power source, such as an engine. A burner for a thermal cycle engine, such as a Stirling cycle engine, should have a high thermal efficiency, low emissions, good cold starting capabilities and a large turndown ratio or wide dynamic range. Stirling cycle machines, including engines and refrigerators, have a long technological heritage, described in detail in Walker, Stirling Engines, Oxford University Press (1980), incorporated herein by reference. High thermal efficiency may be achieved by preheating the air that will be mixed with the fuel in the burner to approximately the Stirling heater head temperature. As discussed previously, low emissions may be achieved by mixing the fuel with the preheated air before burning the fuel in the burner. However, a burner for a thermal cycle engine should also be capable of being ignited and warmed-up with ambient temperature air. Therefore, the burner should be capable of good fuel/air mixing and flame stabilization over a wide range of air temperatures. In addition, the burner should be capable of good fuel/air mixing over a wide range of fuel flows.
In accordance with embodiments of the invention, a liquid fuel burner is provided for combusting a fuel-air mixture. The liquid fuel burner includes a fuel injector for injecting the fuel into the air in a throat of the burner so that the fuel and air mix to form the fuel-air mixture. The fuel injector has a fast acting valve to provide a pulsed flow of fuel and a nozzle coupled to the valve for receiving and atomizing the pulsed flow of fuel. The pulse of atomizing fuel is injected into the burner throat. The burner may include one or more air registers to direct air into the burner throat. The burner further includes a combustion chamber coupled to the throat of the burner for receiving and igniting the fuel air mixture using an igniter. A fuel controller coupled to the fuel supply and the fuel injector governs a rate of fuel delivery by controlling the duration of an opening period of the fast acting valve.
In one embodiment, the fuel controller governs the rate of fuel delivery by varying the frequency of the fast acting valve. Alternatively, the fuel controller may govern the rate of fuel delivery by varying a fuel pressure provided by a fuel pump coupled to the fuel supply. In a further embodiment, the fuel controller includes a pulse width modulated driver to control the frequency and duration of fast acting valve openings. The liquid fuel burner may further include a cooling loop coupled to the fuel supply and the fuel injector for cooling the fuel injector. The valve may be an automotive fuel injector designed for port fuel injection. The nozzle may be a pressure-atomizing oil burner nozzle. The fuel injector may be an automotive gasoline direct injection fuel injector. Alternatively, the fuel injector may be a diesel common rail injector.
In another embodiment, the liquid fuel burner may be used to provide heat to a thermal cycle engine having a heater head for heating a working fluid by conduction. The fuel flow rate may be controlled to maintain a desired heater head temperature. The fuel flow rate is varied by a controller that varies at least one parameter of a control signal to the fuel injector valve based on the desired heater head temperature and measured heater head temperature. The control signal has the following parameters: signal amplitude, frequency and duty cycle.
In another embodiment, the liquid fuel burner further includes a mixing chamber coupled to the fuel injector for mixing the injected fuel and a portion of the air from the air supply before entry into the throat of the burner. The mixing chamber may include a mesh metal surface to absorb and evaporate the fuel. The mixing chamber may have a plurality of openings through which the portion of air enters the mixing chamber. In one embodiment, the mixing chamber is a cylinder aligned with the fuel injector.